Thermal load leveling using anisotropic materials

ABSTRACT

An apparatus for growing a silicon crystal substrate comprising a heat source, an anisotropic thermal load leveling component, a crucible, and a cold plate component is disclosed. The anisotropic thermal load leveling component possesses a high thermal conductivity and may be positioned atop the heat source to be operative to even-out temperature and heat flux variations emanating from the heat source. The crucible may be operative to contain molten silicon in which the top surface of the molten silicon may be defined as a growth interface. The crucible may be substantially surrounded by the anisotropic thermal load leveling component. The cold plate component may be positioned above the crucible to be operative with the anisotropic thermal load leveling component and heat source to maintain a uniform heat flux at the growth surface of the molten silicon.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.13/292,410 filed Nov. 9, 2011, which is herein incorporated by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contractnumber DE-EE0000595 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

Field of the Invention

Embodiments of the present disclosure relate to the field of siliconcrystal growth. More particularly, the present invention relates tothermal load leveling using anisotropic materials to control siliconcrystal growth.

Discussion of Related Art

Demand for solar cells continues to increase as the demand for renewableenergy sources increases. As these demands increase, one goal of thesolar cell industry is to lower the cost/power ratio. Solar energy interms of $/watt is expensive due in part to the cost of manufacturingsolar cells. There are two types of solar cells: silicon and thin film.The majority of solar cells are made from silicon wafers, such as singlecrystal silicon wafers which accounts for the majority of the costassociated with the manufacture of crystalline silicon solar cells. Theefficiency of the solar cell, or the amount of power produced understandard illumination, is limited, in part, by the quality of thiswafer. Currently, the production of these solar wafers can exceed 40% ofthe entire cost of a solar cell. Thus, providing high quality solarwafer production in a cost efficient manner can reduce the overall costof solar energy. It is with respect to these and other considerationsthat the present improvements have been needed.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended asan aid in determining the scope of the claimed subject matter.

Various embodiments are generally directed to silicon crystal growth forsilicon wafer production used to manufacture solar cells. In oneembodiment, there is disclosed an apparatus for growing a siliconcrystal substrate that includes a heat source, an anisotropic thermalload leveling component, a crucible, and a cold plate component. Thecrucible is operative to contain molten silicon in which a top surfaceof the molten silicon is defined as a growth interface. The anisotropicthermal load leveling component has a high thermal conductivity and isdisposed between the heat source and the crucible. The anisotropicthermal load leveling component is operative to even-out temperature andheat flux variations emanating from the heat source. The cold platecomponent is positioned above the crucible to absorb heat from themolten silicon to crystallize the molten silicon into a silicon crystalsubstrate.

In another embodiment, a method of growing a silicon crystal substratecomprises filling a crucible with molten silicon in which a portion ofthe top surface of the molten silicon defines a growth surface. Thecrucible and the molten silicon within the crucible are heated using aheat source. The heat from the heat source incident on a surface of thecrucible is regulated via a passive thermal load leveling anisotropicmaterial disposed between the heat source and the crucible. A uniformheat flux at the growth surface of the molten silicon is maintained bycooling an area above the growth surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of an FSM apparatus, according to anembodiment of the present disclosure.

FIG. 2 illustrates a block diagram of an FSM apparatus, according toanother embodiment of the present disclosure.

FIG. 3 illustrates a logic flow diagram associated with the apparatusshown in FIG. 1, according to an embodiment of the present disclosure.

FIG. 4 illustrates a logic flow diagram associated with the apparatusshown in FIG. 2, according to another embodiment of the presentdisclosure.

FIG. 5 illustrates a graph depicting a figure of merit (FM)relationship.

FIG. 6 illustrates an embodiment of an exemplary computer system 600suitable for implementing various embodiments of the present disclosure.

DESCRIPTION OF EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention, however, may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, like numbers refer to like elements throughout.

Various embodiments are directed to a thermal load leveling componentdisposed between a heat source and a crucible containing molten siliconused in silicon wafer manufacturing to aid in leveling out temperatureand heat flux variations produced by the heat source. In someembodiments, a pump apparatus and a baffle structure may also beutilized within the crucible to promote uniform flow of the moltensilicon leading to further leveling of temperature and heat fluxvariations.

When growing silicon crystal substrates such as, for instance, siliconphotovoltaic substrates for use in solar cells, a floating siliconmethod (FSM) may be used. By way of background, silicon crystal growthusing FSM is similar to how ice forms on a pond when the air temperaturedrops below the freezing point for water. A solid ice surface will beginto form on the surface of the pond and “grow” downward over time intothe warmer water based on the air temperature at the surface of the pondand the current water temperature.

FSM silicon crystal growth generally comprises a crucible filled withmolten silicon that is heated from below and cooled from above. Thecooling component causes the molten silicon to begin solidifying at itstop surface which is referred to as the growth region. The siliconcrystal substrate growth extends downward into the molten silicon. Thegrowth rate of the silicon crystal is therefore dependent on thetemperature of the molten silicon and the cooling component. As thesilicon grows vertically downward, it is also pulled or transported in ahorizontal direction out of the crucible and away from the heating andcooling sources. Through precise manipulation of temperature gradientsaffecting the molten silicon, the silicon crystal can grow at a constantrate and can be pulled or transported at a constant rate to ensure arelatively uniform thickness for the silicon crystal substrate. Thus, itis advantageous to maintain horizontally uniform temperature conditionsin the growth region.

Optimally, the vertical growth rate should be as uniform as possible.This may be accomplished by maintaining a near constant vertical heatflux relative to the growth surface. The temperature at the top surfaceof the silicon crystal substrate should be maintained at a temperaturethat is less than the temperature of the growth surface in order toremove energy. The temperature difference should be relatively small soas to minimize stress within the silicon crystal substrate. Maintainingthis condition may be troublesome, however, when the heat sourceexperiences spatial temperature variations. In particular, the heatsource used to heat the molten silicon may generate temperaturegradients across the molten silicon. Introducing a thermal load levelingcomponent at the proper point in an FSM system can significantlyeven-out temperature and heat flux variations present in the heat sourceand consequently the temperature of the molten silicon.

Generally, heat flux is the rate of heat energy transfer through a givensurface. In the International System of Units (SI), heat flux ismeasured in (W/m²) and is the rate of thermal energy transferred perunit area. The measurement of heat flux is most often done by measuringa temperature difference over a piece of material with known thermalconductivity which refers to a material's ability to conduct heat. Heattransfer across materials of high thermal conductivity occurs at afaster rate than across materials of low thermal conductivity. Materialsof high thermal conductivity may be used in heat source applications.The SI derived unit of heat rate is joules per second, or watt. The heatrate is a scalar quantity, while heat flux is a vector quantity.

The thermal load leveling component used in an FSM system to even-outtemperature and heat flux variations should be highly anisotropic sincethe heat energy is directed toward the molten silicon. Anisotropy is theproperty of being directionally dependent, as opposed to isotropy, whichimplies identical properties in all directions. Anisotropy can bedefined as a difference, when measured along different axes, in amaterial's physical or mechanical properties including thermalconductivity. Temperature variations in the direction of high-thermalconductivity in an anisotropic material tend to be eliminated becausethere is rapid heat transfer in that direction. This can aid in levelingout temperature and heat flux variations caused by a non-uniform heatsource. This leads to growth of more uniform, higher quality siliconcrystal substrates used to manufacture solar cells or other devices.

FIG. 1 illustrates a block diagram of a floating silicon method (FSM)apparatus 100, according to an embodiment of the invention. The FSMapparatus 100 is generally directed to growing a silicon crystalsubstrate 120 from molten silicon 116. Apparatus 100 includes a heatsource 110 which may be comprised of, for example, graphite. In oneembodiment, a graphite heat source 110 may be approximately 2 mm thickand possess a thermal conductivity (k) value of approximately 4 wattsper meter-kelvin (W/mK). A crucible 114 is adapted to contain the moltensilicon 116 and may be comprised of quartz. In one exemplary embodiment,crucible 114 may be approximately 5 mm thick and possess a thermalconductivity (k) value of approximately 4 W/mK.

An anisotropic thermal load leveling component 112 is disposed betweenthe heat source 110 and crucible 114. The anisotropic load levelingcomponent 112 at least partially surrounds crucible 114 and may becomprised of pyrolytic graphite which is a highly anisotropic material.In pyrolytic graphite, carbon atoms form a structure that in onedirection is characterized by planar layers of hexagonally arrangedcarbon atoms and in a direction perpendicular to the planar layerscomprises randomly oriented atoms. This causes a high thermalconductivity (k) in the direction of the planar layers, but a very lowthermal conductivity in the perpendicular direction. Pyrolytic graphitemay also be characterized as a passive component because it requires noadditional energy or control to obtain and maintain its anisotropic highthermal conductivity properties. In one embodiment, the pyrolyticgraphite may be approximately 10 mm thick and possess a thermalconductivity (k) value of approximately 300 W/mK in the planar directionand only 1 w/mK in the perpendicular direction.

A cold plate component 118 such as, for instance, silicon carbide may bepositioned above the crucible 114 to absorb heat from the molten silicon116 such that a growth interface forms therebetween. This cold platecomponent 118 may absorb heat using radiative heat transfer or acombination of radiative and convective heat transfer, for example. Themolten silicon 116 crystallizes and “grows” in a downward direction assymbolized by the V_(growth) arrow in FIG. 1 to form a silicon crystalsubstrate 120. The heat flowing through the molten silicon 116 isradiated from the top surface to the cold plate component 118 which actsas a heat sink to the radiation. Thus, a lower cold plate temperatureproduces a larger growth rate V_(growth) of the silicon crystalsubstrate 120 while for a given cold plate temperature a larger heatflow rate through the silicon produces a smaller growth rate V_(growth).Thus, the growth rate value V_(growth) is determined by a balance of theheat flow through the melt and the amount of heat absorbed by the coldplate component 118 from the molten silicon 116.

By way of an illustrative example, a 2 mm thick graphite heat source 110having a thermal conductivity of 4 W/mK heats a 10 mm anisotropicthermal load leveling component 112 comprising a pyrolytic graphitematerial and having a thermal conductivity of 300 W/mK in the planar or“x” direction and 1 W/mK in the perpendicular or “y” direction. The “x”and “y” axes are labeled in FIG. 1 for the anisotropic thermal loadleveling component 112. It should be noted that the “y” direction on anygiven point of the anisotropic thermal load leveling component 112 isalways perpendicular to the surface of the crucible 114. The anisotropicthermal load leveling component 112 surrounds a 5 mm thick crucible 114which may comprise a quartz material and have a thermal conductivity of4 W/mK. The crucible 114 contains a 10 mm depth of molten silicon 116having a thermal conductivity of 64 W/mK. A cold plate component 118 ispositioned above the molten silicon 116 to create a growth interface atthe top surface of the molten silicon 116 where the molten silicon 116can begin to crystallize and “grow” into a silicon crystal substrate 120in a downward direction as symbolized by the V_(growth) arrow in FIG. 1.

For this example, the desired thickness (S_(y) in FIG. 1) of the siliconcrystal substrate is 100 μm. The cold plate draws a uniform heat flux of10 kW/m² and the temperature of the molten silicon at the bottom of thecrucible is 1687K assuming the growth interface is at the solidificationtemperature of silicon, 1685K. Based on the characteristics describedabove, the growth rate, V_(growth), is 10 μm per second. Thus, thesilicon crystal substrate will reach the desired thickness of 100 μm in10 seconds. The pull-rate, V_(x), should be approximately 2 cm/s totraverse the 20 cm growth area such that the silicon crystal substratethickness (S_(y)) will be 100 μm when it is pulled or transported awayfrom the growth interface and onto a support table 122 where it can becut into sheets. In this manner, the anisotropic thermal load levelingcomponent 112 keeps the heat flux to the growth interface horizontallyuniform so that a uniform vertical growth rate and sheet thickness canbe obtained. In an alternate embodiment, the substrate thickness (S_(y))will be grown to slightly larger than 100 μm to compensate for anythickness loss due to the molten silicon as the silicon crystalsubstrate 120 is pulled or transported from the FSM apparatus 100. Thismay enable the silicon crystal substrate 120 to have a substratethickness (S_(y)) of 100 μm when it reaches the support table 122.

FIG. 2 illustrates a block diagram according to an alternativeembodiment of the present disclosure. The FSM apparatus 200 issubstantially similar to that described with reference to FIG. 1 withthe exception of a pump 215 and baffle structure 217. In particular,apparatus 200 includes an anisotropic thermal load leveling component112 disposed between heat source 110 and crucible 114. Molten silicon116 is deposited in crucible 114. The molten silicon 116 passes throughpump 215 which is operative to cause the molten silicon 116 to flowabout baffle structure 217 in a direction as indicated by arrows A. Bycausing the molten silicon within the crucible 114 to flow about bafflestructure 217, the thermal gradient variations can be further reduced.Consequently, the reduction in thermal gradient variations of the moltensilicon 116 within crucible 114 provides for more consistentcrystallization using cold plate component 118. In this manner, higherquality silicon substrates are produced in a more cost efficient andreliable manner. In addition, the constant flow serves to substantiallyuniformly disperse any impurities within the molten silicon such thatthere are no isolated occurrences of high concentrations of impuritiesthat end up in the finished product.

Included herein is one or more flow charts representative of exemplarymethodologies for performing novel aspects of the disclosed structure.While, for purposes of simplicity of explanation, the one or moremethodologies shown herein, for example, in the form of a flow chart orflow diagram, are shown and described as a series of acts, it is to beunderstood and appreciated that the methodologies are not limited by theorder of acts, as some acts may, in accordance therewith, occur in adifferent order and/or concurrently with other acts from that shown anddescribed herein. Moreover, not all acts illustrated in a methodologymay be required for a novel implementation.

FIG. 3 illustrates a logic flow diagram according to an embodiment ofthe invention. The logic flow 300 may be representative of some or allof the operations executed by apparatuses 100 and/or 200 describedherein.

In the illustrated embodiment shown in FIG. 3, the logic flow 300 mayfill a crucible 114 with molten silicon 116 at block 302. For example,the crucible 114 may be surrounded by a passive material such asanisotropic thermal load leveling component 112 having a high thermalconductivity. A portion of the top surface of the molten silicon 116 isa growth surface. The embodiments are not limited to this example.

The logic flow 300 may heat the anisotropic thermal load levelingcomponent 112 using a heat source 110 at block 304. For example, theheat source 110 may be comprised of graphite. The embodiments are notlimited to this example.

The logic flow 300 may create a cool zone above the growth interface ofthe molten silicon 116 at block 306. For example, a cold plate component118 such as, for instance, silicon carbide may be positioned just abovethe molten silicon 116. The cold plate component 118 may cause themolten silicon 116 to begin solidifying at the growth interface bylowering the temperature at the growth surface to the equilibriumsolidification temperature for silicon. The embodiments are not limitedto this example.

The logic flow 300 may maintain a uniform heat flux at the growthsurface of the molten silicon 116 at block 308. For example, theanisotropic thermal load leveling component 112 uniformly distributesthe heat it absorbs from heat source 104 to crucible 114. In turn, themolten silicon 116 within crucible 114 is uniformly maintained such thatthe heat flux at the growth surface is also uniform. The uniformity mayensure a uniform growth rate for the silicon crystal substrate 120. Theembodiments are not limited to this example.

The logic flow 300 may pull or otherwise transport the growing siliconcrystal substrate 120 at a constant pull rate away from the crucible 114at block 310. For example, the growing silicon crystal substrate 120 maybe horizontally pulled at a constant rate (V_(x)) away from the coldplate component 118 and out of the crucible 114. The pull rate forsilicon crystal substrate 120 may correspond with the growth rate of thesilicon crystal substrate 120 such that the thickness (S_(y)) of thesilicon crystal substrate 120 is uniform. The silicon crystal substrate120 may then rest on a support table 122. The embodiments are notlimited to this example.

FIG. 4 illustrates a logic flow diagram according to an embodiment ofthe invention. The logic flow 400 may be representative of some or allof the operations executed by apparatuses 100 and/or 200 describedherein.

In the illustrated embodiment shown in FIG. 4, the logic flow 400 mayfill a crucible 114 with molten silicon 116 at block 402. For example,the crucible 114 may be surrounded by an anisotropic thermal loadleveling component 112 having a high thermal conductivity. A portion ofthe top surface of the molten silicon 116 is a growth surface. Theembodiments are not limited to this example.

The logic flow 400 may heat the anisotropic thermal load levelingcomponent 112 using a heat source 110 at block 404. For example, theheat source 110 may be comprised of graphite. The embodiments are notlimited to this example.

The logic flow 400 may pump the molten silicon about a baffle structurepositioned within the crucible 114 at block 406. For example, a pump 215may pump the molten silicon 116 about a baffle structure 217 disposedwithin the crucible 114 has been included. Keeping the molten siliconflowing using the pump 215 may further reduce temperature gradients ofthe molten silicon 116 leading to a higher quality and more consistentsilicon crystal substrate 120 growth. In addition, the constant flow mayserve to substantially uniformly disperse any impurities within themolten silicon 116 such that there are no isolated occurrences of highconcentrations of impurities that end up in the finished product. Theembodiments are not limited to this example.

The logic flow 400 may create a cool zone above the growth interface ofthe molten silicon 116 at block 408. For example, a cold plate component118 such as, for instance, silicon carbide may be positioned just abovethe molten silicon 116. The cold plate component 118 may cause themolten silicon 116 to begin solidifying at the growth interface bylowering the temperature at the growth surface to the equilibriumsolidification temperature for silicon. The embodiments are not limitedto this example.

The logic flow 400 may maintain a uniform heat flux at the growthsurface of the molten silicon 116 at block 410. For example, theanisotropic thermal load leveling component 112 uniformly distributesthe heat it absorbs from heat source 104 to crucible 114. In turn, themolten silicon 116 within crucible 114 is uniformly maintained such thatthe heat flux at the growth surface is also uniform. The uniformity mayensure a uniform growth rate for the silicon crystal substrate 120. Theembodiments are not limited to this example.

The logic flow 400 may pull or otherwise transport the growing siliconcrystal substrate 120 at a constant pull rate away from the crucible 114at block 412. For example, the growing silicon crystal substrate 120 maybe horizontally pulled at a constant rate (V_(x)) away from the coldplate component 118 and out of the crucible 114. The pull rate forsilicon crystal substrate 120 may correspond with the growth rate of thesilicon crystal substrate 120 such that the thickness (S_(y)) of thesilicon crystal substrate 120 is uniform. The silicon crystal substrate120 may then rest on a support table 122. The embodiments are notlimited to this example.

FIG. 5 illustrates a graph depicting a figure of merit (FM)relationship. The FM quantifies the uniformity of the heat flux to thelower growth surface (e.g., bottom of growing silicon crystal substrate)given a non-uniform heat source 110. It is defined as the differencebetween the maximum and minimum heat flux from the heat source surface110 divided by the difference between the maximum and minimum heat fluxto the lower growth surface. It is advantageous to have an FM as largeas possible as this indicates that the variation in heat flux to thegrowth interface is small.

FIG. 5 illustrates the highly nonlinear results of calculations forpyrolytic graphite. The calculations assumed a linear variation in heatflux from the heat source surface. This heat then passed through a layerof pyrolytic graphite ranging in thickness from zero to 20 mm, thenthrough 5 mm of quartz, and finally through 10 mm of molten silicon tothe silicon crystal substrate. Each layer was assumed to be 20 cm inlength. When the thickness of the pyrolytic graphite is 20 mm the FM is204 indicating that the variation in heat flux to the substrate was 204times less than the variation in heat flux from the heater.

FIG. 6 illustrates an embodiment of exemplary computer system 600suitable for implementing the FSM system 100 of FIG. 1 and/or FSM system200 of FIG. 2. As shown in FIG. 6, the computer system 600 comprises aprocessing component 605, a system memory 610 storing a load levelingcontrol application 615, a user interface component 620, a networkinterface 625, and an FSM interface 630. The processing component 605can be any of various commercially available processors including dualmicroprocessors and other multi processor architectures. The processingcomponent 605 is communicatively coupled with the other components. Inaddition, the computer system 600 may be communicatively coupled with anexternal network 650 via network interface 625.

The computer system 600, via load leveling control application 615, isoperative to receive substrate thickness S_(y) measurements from FSMinterface 630. These substrate thickness measurements S_(y) may beobtained as the silicon crystal substrate is pulled from crucible 114and may be collected based on sampling times and/or based on the lengthof substrate pulled from the growth interface. The control application615 is operative to compare this thickness measurement to predeterminedacceptable thickness values stored in memory 610 corresponding to adesired wafer geometry. If the collected thickness values S_(y) are notwithin tolerance levels of the predetermined acceptable thickness valuesstored in memory 610, control application 615 may execute controlinstructions to adjust the power supplied to heat source 110 and coldplate 118 to modify the temperatures thereof and/or to adjust the pullrate of the silicon crystal substrate pulled from the growth interfaceonto support table 120. In addition, control application 615 may alsoalert an operator to adjust the amount of molten silicon 116 withincrucible 114. This measurement and parameter modification process may berepeated until a desired thickness of silicon crystal substrate 120 isachieved. Moreover, the use of the anisotropic thermal load levelingcomponent 112 disposed between the heat source 110 and crucible 114which provides reduces heat flux variations incident on the moltensilicon from the heat source may also be considered when implementingthese adjustments. Each of these adjustments may be implemented ascontrol instruction executed by load leveling control application 615.In addition, these control instructions may be pre-programmed or may beinput by a human operator via a user interface component 620. In thismanner, feedback including, but not limited to, various temperature,pull rate, and thickness measurements may be monitored and returned tothe computer system 600 for analysis and processing by the load levelingcontrol application 615 to produce a silicon substrate having a desiredthickness.

As used herein, the terms “system” and “device” and “component” areintended to refer to a computer-related entity, either hardware, acombination of hardware and software, software, or software inexecution, examples of which are provided by the exemplary computingarchitecture 600. For example, a component can be, but is not limited tobeing, a process running on a processor, a processor, a hard disk drive,multiple storage drives (of optical and/or magnetic storage medium), anobject, an executable, a thread of execution, a program, and/or acomputer. By way of illustration, both an application running on aserver and the server can be a component. One or more components canreside within a process and/or thread of execution, and a component canbe localized on one computer and/or distributed between two or morecomputers. Further, components may be communicatively coupled to eachother by various types of communications media to coordinate operations.The coordination may involve the uni-directional or bi-directionalexchange of information. For instance, the components may communicateinformation in the form of signals communicated over the communicationsmedia. The information can be implemented as signals allocated tovarious signal lines. In such allocations, each message is a signal.Further embodiments, however, may alternatively employ data messages.

While the present invention has been disclosed with reference to certainembodiments, numerous modifications, alterations and changes to thedescribed embodiments are possible without departing from the sphere andscope of the present invention, as defined in the appended claims.Accordingly, it is intended that the present invention not be limited tothe described embodiments, but that it has the full scope defined by thelanguage of the following claims, and equivalents thereof.

What is claimed is:
 1. A method of growing a silicon crystal substratecomprising: filling a crucible with molten silicon in which a portion ofthe top surface of the molten silicon defines a growth surface; heatingthe crucible using a heat source; regulating the heat from the heatsource incident on a surface of the crucible via a passive thermal loadleveling anisotropic material disposed between the heat source and thecrucible; and maintaining a uniform heat flux at the growth surface ofthe molten silicon by cooling an area above the growth surface.
 2. Themethod of claim 1 further comprising pulling the growing silicon crystalsubstrate at a constant pull rate away from the crucible.
 3. The methodof claim 2 further comprising: measuring a thickness of the siliconcrystal substrate; and determining if the measured thickness of thesilicon substrate is within acceptable tolerance values.
 4. The methodof claim 3 further comprising modifying an amount of heat from the heatsource incident on the surface of the crucible if the measured thicknessof the silicon substrate is not within the acceptable tolerance values.5. The method of claim 3 further comprising modifying a rate of coolingthe area above the growth surface if the measured thickness of thesilicon substrate is not within the acceptable tolerance values.
 6. Themethod of claim 3 further comprising modifying the pull rate away fromthe crucible if the measured thickness of the silicon substrate is notwithin the acceptable tolerance values.
 7. The method of claim 3 furthercomprising modifying the filling of the crucible with an amount ofmolten silicon if the measured thickness of the silicon substrate is notwithin the acceptable tolerance values.
 8. The method of claim 1 furthercomprising pumping the molten silicon about a baffle structurepositioned within the crucible.
 9. The method of claim 1 wherein thepassive thermal load leveling anisotropic material is comprised ofpyrolytic graphite.